<pre class='metadata'>
Title: std::expected
Shortname: P0323
Revision: 5
Audience: LWG
Status: P
Group: WG21
URL: https://wg21.link/P0323r5
!Source: <a href="https://github.com/jfbastien/papers/blob/master/source/P0323r5.bs">github.com/jfbastien/papers/blob/master/source/P0323r5.bs</a>
Editor: Vicente Botet, Nokia, vicente.botet@nokia.com
Editor: JF Bastien, Apple, jfbastien@apple.com
Abstract: Utility class to represent expected object: wording and open questions.
Date: 2018-02-08
Markup Shorthands: markdown yes
</pre>

This paper revises [[P0323r4]] by applying feedback obtained from LEWG and
EWG. The previous paper contains motivation, design rationale, implementability
information, sample usage, history, alternative designs and related types. This
update only contains wording and open questions because its purpose is twofold:

  * Present appropriate wording for inclusion in the Library Fundamentals TS v3.
  * List open questions which the TS should aim to answer.

Wording {#word}
=======

Below, substitute the `�` character with a number or name the editor finds
appropriate for the sub-section.

�.� Unexpected objects [*unexpected*] {#unexpected}
-------------------------------------

�.�.1 General [*unexpected.general*] {#unexpected.general}
------------------------------------

This subclause describes class template `unexpected` that 
represents unexpected objects.

�.�.2 Header `<experimental/unexpected>` synopsis [*unexpected.synop*] {#unexpected.synop}
----------------------------------------------------------------------

```c++
namespace std {
namespace experimental {
inline namespace fundamentals_v3 {
    // �.�.3, Unexpected object type
    template <class E>
      class unexpected;

    // �.�.4, Unexpected equality operators
    template <class E1, class E2>
        constexpr bool
        operator==(const unexpected<E1>&, const unexpected<E2>&);
    template <class E1, class E2>
        constexpr bool
        operator!=(const unexpected<E1>&, const unexpected<E2>&);
        
    // �.�.5,  Specialized algorithms 
    void swap(unexpected& x, unexpected& y) noexcept(noexcept(x.swap(y)));;

}}}
```

A program that necessitates the instantiation of template `unexpected` for a 
non-object type or an object type cv-qualified is ill-formed.

�.�.3 Unexpected object type [*unexpected.object*] {#unexpected.object}
--------------------------------------------------

```c++
template <class E>
class unexpected {
public:
    unexpected() = delete;
    constexpr unexpected(const unexpected&) = default;
    constexpr unexpected(unexpected&&) = default;    
    constexpr unexpected& operator=(const unexpected&) = default;
    constexpr unexpected& operator=(unexpected&&) = default;    
    
    template <class Err>
    constexpr explicit unexpected(Err&&);

    template <class Err>
    constexpr EXPLICIT unexpected(const unexpected<Err>&);
    template <class Err>
    constexpr EXPLICIT unexpected(unexpected<Err>&&);
    
    constexpr const E& value() const& noexcept;
    constexpr E& value() & noexcept;
    constexpr const E&& value() const&& noexcept;
    constexpr E&& value() && noexcept;
private:
    E val; // exposition only
};
```


```c++
template <class Err>
constexpr explicit unexpected(Err&& e);
```

*Effects*: Initializes `val` as if direct-non-list-initializing an
object of type `E` with the expression `std::forward<Err>(e)`.

*Remark*: This constructor participates in overload resolution if and only if `std::is_constructible_v<E, Err&&>`

```c++
template <class Err>
constexpr EXPLICIT unexpected(const unexpected<Err>& e);
```

*Effects*: Initializes `val` as if direct-non-list-initializing an
object of type `E` with the expression `e.val`.

*Remarks*: This constructor participates in overload resolution if and only if
`std::is_constructible_v<E, Err>`. This constructor is `explicit` if and only if
`std::is_convertible_v<Err, E>` is `false`.

```c++
template <class Err>
constexpr EXPLICIT unexpected(unexpected<Err> && e);
```

*Effects*: Initializes `val` as if direct-non-list-initializing an
object of type `E` with the expression `std::move(e.val)`.

*Remarks*: This constructor participates in overload resolution if and only if
`std::is_constructible_v<E, Err&&>`. This constructor is `explicit` if and only if
`std::is_convertible_v<Err&&, E>` is `false`

```c++
constexpr const E& value() const &;
constexpr E& value() &;
```

*Returns*: `val`.

```c++
constexpr E&& value() &&;
constexpr E const&& value() const&&;
```

*Returns*: `std::move(val)`.



�.�.4 Unexpected equality operators [*unexpected.relational_op*] {#unexpected.relational_op}
------------------------------------------------------------------

```c++
template <class E, class G>
    constexpr bool operator==(const unexpected<E>& x, const unexpected<G>& y);
```

*Effects*: Equivalent to `return x.value() == y.value()`.

```c++
template <class E, class G>
    constexpr bool operator!=(const unexpected<E>& x, const unexpected<G>& y);
```

*Effects*: Equivalent to `return x.value() != y.value()`.

�.� Expected objects [expected] {#expected}
-------------------------------

�.�.1 In general [*expected.general*] {#expected.general}
-------------------------------------

This subclause describes class template `expected` that represents expected
objects. An `expected<T, E>` object holds an object of type `T` or 
an object of type `unexpected<E>` and manages the lifetime of the contained
object. 

�.�.2 Header `<experimental/expected>` synopsis [*expected.synop*] {#expected.synop}
------------------------------------------------------------------

```c++
namespace std {
namespace experimental {
inline namespace fundamentals_v3 {
    // �.�.4, Expected for object types
    template <class T, class E>
        class expected;

    // �.�.5, unexpect tag
    struct unexpect_t {
       explicit unexpect_t() = default;
    };
    inline constexpr unexpect_t unexpect{};

    // �.�.6, class bad_expected_access
    template <class E>
       class bad_expected_access;

    // �.�.7, Specialization for void
    template <>
       class bad_expected_access<void>;

    // �.�.8, Expected equality operators
    template <class T1, class E1, class T2, class E2>
        constexpr bool operator==(const expected<T1, E1>& x, const expected<T2, E2>& y);
    template <class T1, class E1, class T2, class E2>
        constexpr bool operator!=(const expected<T1, E1>& x, const expected<T2, E2>& y);

    // �.�.9, Comparison with T
    template <class T, class E, class U>
      constexpr bool operator==(const expected<T, E>&, const U&);
    template <class U, class T, class E>
      constexpr bool operator==(const U&, const expected<T, E>&);
    template <class T, class E, class U>
      constexpr bool operator!=(const expected<T, E>&, const U&);
    template <class U, class T, class E>
      constexpr bool operator!=(const U&, const expected<T, E>&);
      
    // �.�.10, Comparison with unexpected<E>
    template <class T, class E, class G>
      constexpr bool operator==(const expected<T, E>&, const unexpected<G>&);
    template <class G, class T, class E>
      constexpr bool operator==(const unexpected<G>&, const expected<T, E>&);
    template <class T, class E, class G>
      constexpr bool operator!=(const expected<T, E>&, const unexpected<G>&);
    template <class G, class T, class E>
      constexpr bool operator!=(const unexpected<G>&, const expected<T, E>&);

    // �.�.11, Specialized algorithms
    template <class T, class E>
    void swap(expected<T, E>&, expected<T, E>&) noexcept(see below);

}}}
```

A program that necessitates the instantiation of template `expected<T, E>`
for a reference type or for possibly cv-qualified types `in_place_t`,
`unexpect_t` or `unexpected<E>` for the `T` parameter or 
for a reference type `E` or for possibly cv-qualified `void` type for the `E` parameter
is ill-formed.

�.�.3 Definitions [*expected.defs*] {#expected.defs}
-----------------------------------

An instance of `expected<T, E>` is said to be valued if it contains a object of
type `T`. An instance of `expected<T, E>` is said to be unexpected if it
contains an object of type `unexpected<E>`.


�.�.4 expected for object types [*expected.object*] {#expected.object}
---------------------------------------------------

```c++
template <class T, class E>
class expected {
public:
    using value_type = T;
    using error_type = E;
    using unexpected_type = unexpected<E>;

    template <class U>
    using rebind = expected<U, error_type>;

    // �.�.4.1, constructors
    constexpr expected();
    constexpr expected(const expected&);
    constexpr expected(expected&&) noexcept(see below);
    template <class U, class G>
        EXPLICIT constexpr expected(const expected<U, G>&);
    template <class U, class G>
        EXPLICIT constexpr expected(expected<U, G>&&);

    template <class U = T>
        EXPLICIT constexpr expected(U&& v);

    template <class... Args>
        constexpr explicit expected(in_place_t, Args&&...);
    template <class U, class... Args>
        constexpr explicit expected(in_place_t, initializer_list<U>, Args&&...);
    template <class G = E>
        constexpr expected(const unexpected<G>&);
    template <class G = E>
        constexpr expected(unexpected<G> &&);
    template <class... Args>
        constexpr explicit expected(unexpect_t, Args&&...);
    template <class U, class... Args>
        constexpr explicit expected(unexpect_t, initializer_list<U>, Args&&...);

    // �.�.4.2, destructor
    ~expected();

    // �.�.4.3, assignment
    expected& operator=(const expected&);
    expected& operator=(expected&&) noexcept(see below);
    template <class U = T> expected& operator=(U&&);
    template <class G = E>
        expected& operator=(const unexpected<G>&);
    template <class G = E>
        expected& operator=(unexpected<G>&&);

    template <class... Args>
        void emplace(Args&&...);
    template <class U, class... Args>
        void emplace(initializer_list<U>, Args&&...);

    // �.�.4.4, swap
    void swap(expected&) noexcept(see below);

    // �.�.4.5, observers
    constexpr const T* operator ->() const;
    constexpr T* operator ->();
    constexpr const T& operator *() const&;
    constexpr T& operator *() &;
    constexpr const T&& operator *() const &&;
    constexpr T&& operator *() &&;
    constexpr explicit operator bool() const noexcept;
    constexpr bool has_value() const noexcept;
    constexpr const T& value() const&;
    constexpr T& value() &;
    constexpr const T&& value() const &&;
    constexpr T&& value() &&;
    constexpr const E& error() const&;
    constexpr E& error() &;
    constexpr const E&& error() const &&;
    constexpr E&& error() &&;
    template <class U>
        constexpr T value_or(U&&) const&;
    template <class U>
        constexpr T value_or(U&&) &&;

private:
    bool has_val; // exposition only
    union
    {
        value_type val; // exposition only
        unexpected_type unexpect; // exposition only
    };
};
```

Any instance of `expected<T, E>` at any given time either contains a value of
type `T` or a value of type `unexpected<E>` within their own storage.
Implementations are not permitted to use additional storage, such as dynamic 
memory, to allocate the object of type `T` or the object of type `unexpected<E>`.
These objects shall be allocated in a region of the `expected<T, E>` storage
suitably aligned for the types `T` and `unexpected<E>`. Members `has_val`, `val`
and `unexpect` are provided for exposition only. `has_val` indicates whether the
object `T` has been initialized (and not yet destroyed) or an object of type
`unexpected<E>`has been initialized (and not yet destroyed).

`T` shall be `void` or shall be an object type and shall satisfy the requirements
of `Destructible` (Table 27).

`E` shall be object type and shall satisfy the requirements of `Destructible` (Table 27).


�.�.4.1 Constructors [*expected.object.ctor*] {#expected.object.ctor}
---------------------------------------------

```c++
constexpr expected();
```

*Effects*: Value-initializes the contained object as if direct-non-list-initializing an
object of type `T` with the expression `T{}` (if `T` is not `void`).

*Postconditions*: `bool(*this)`.

*Throws*: Any exception thrown by the operations specified in the effect clause.


*Remarks*: If value-initialization of `T` is a constexpr constructor or `T` is
`void` this constructor shall be constexpr. This constructor shall be defined as
deleted unless `is_default_constructible_v<T>` or `T` is `void`.

```c++
constexpr expected(const expected& rhs);
```

*Effects*: If `bool(rhs)`, initializes `val` as if
direct-non-list-initializing an object of type `T` with the expression `*rhs`
(if `T` is not `void`).

If `!bool(rhs)`, initializes `val` as if
direct-non-list-initializing an object of type `unexpected<E>` with the
expression `unexpected(rhs.error())`.

*Postconditions*: `bool(rhs) == bool(*this)`.

*Throws*: Any exception thrown by operations specified in the effect clause.

*Remarks*: This constructor shall be defined as deleted unless
`is_copy_constructible_v<T>` or `T` is `void` and
`is_copy_constructible_v<E>`. If `is_trivially_copy_constructible_v<T>` is
`true` or `T` is `void` and `is_trivially_copy_constructible_v<E>` is `true`,
this constructor shall be a constexpr constructor.


```c++
constexpr expected(expected && rhs) noexcept(see below);
```

*Effects*: If `bool(rhs)`, initializes `val` as if
direct-non-list-initializing an object of type `T` with the expression
`std::move(*rhs)` (if `T` is not `void`).

If `!bool(rhs)`, initializes `val` as if
direct-non-list-initializing an object of type `unexpected<E> ` with the
expression `unexpected(std::move(rhs.error()))`.

`bool(rhs)` is unchanged.

*Postconditions*: `bool(rhs) == bool(*this)`.

*Throws*: Any exception thrown by operations specified in the effect clause.

*Remarks*: The expression inside `noexcept` is equivalent to:
`is_nothrow_move_constructible_v<T>` or `T `is `void` and
`is_nothrow_move_constructible_v<E>`. This constructor shall not participate in
overload resolution unless `is_move_constructible_v<T>` and
`is_move_constructible_v<E>`. If `is_trivially_move_constructible_v<T>` is
`true` or `T` is `void` and `is_trivially_move_constructible_v<E>` is `true`,
this constructor shall be a constexpr constructor.


```c++
    template <class U, class G>
    EXPLICIT constexpr expected(const expected<U, G>& rhs);
```

*Effects*: If `bool(rhs)`, initializes `val` as if
direct-non-list-initializing an object of type `T` with the expression `*rhs`
(if `T` is not `void`).

If `!bool(rhs)` initializes `unexpect` as if
direct-non-list-initializing an object of type `unexpected<E>` with the
expression `unexpected(rhs.error())`.

*Postconditions*: `bool(rhs) == bool(*this)`.

*Throws*: Any exception thrown by operations specified in the effect clause.

*Remarks*: This constructor shall not participate in overload resolution unless:
`T` and `U` are `void` or

- `is_constructible_v<T, const U&>` is `true` ,
- `is_constructible_v<E, const G&>` is `true`,
- `is_constructible_v<T, expected<U, G>&>` is `false`,
- `is_constructible_v<T, expected<U, G>&&>` is `false`,
- `is_constructible_v<T, const expected<U, G>&>` is `false`,
- `is_constructible_v<T, const expected<U, G>&&>` is `false`,
- `is_convertible_v<expected<U, G>&, T>` is `false`,
- `is_convertible_v<expected<U, G>&&, T>` is `false`,
- `is_convertible_v<const expected<U, G>&, T>` is `false` and
- `is_convertible_v<const expected<U, G>&&, T>` is `false`.


The constructor is explicit if and only if `T` is not `void` and
`is_convertible_v<U const&, T>` is `false` or `is_convertible_v<const G&, E>` is
`false`.


```c++
template <class U, class G>
    EXPLICIT constexpr expected(expected<U, G>&& rhs);
```

*Effects*: If `bool(rhs)` initializes `val` as if
direct-non-list-initializing an object of type `T` with the expression
`std::move(*rhs)` or nothing if `T` is `void`.

If `!bool(rhs)`, initializes `unexpect` as if
direct-non-list-initializing an object of type `unexpected<E> ` with the
expression `unexpected(std::move(rhs.error()))`. 

`bool(rhs)` is unchanged.

*Postconditions*: `bool(rhs) == bool(*this)`.

*Throws*: Any exception thrown by operations specified in the effect clause.

*Remarks*: This constructor shall not participate in overload resolution unless:
`T` and `U` are `void` or 

- `is_constructible_v<T, U>` is `true`,
- `is_constructible_v<E, G>` is `true`,
- `is_constructible_v<T, expected<U, G>&>` is `false`,
- `is_constructible_v<T, expected<U, G>&&>` is `false`,
- `is_constructible_v<T, const expected<U, G>&>` is `false`,
- `is_constructible_v<T, const expected<U, G>&&>` is `false`,
- `is_convertible_v<expected<U, G>&, T>` is `false`,
- `is_convertible_v<expected<U, G>&&, T>` is `false`,
- `is_convertible_v<const expected<U, G>&, T>` is `false`, and
- `is_convertible_v<const expected<U, G>&&, T>` is `false`.

The constructor is explicit if and only if `is_convertible_v<U, T>` is `false`
or `is_convertible_v<G, E>` is `false`.

```c++
template <class U = T>
    EXPLICIT constexpr expected(U&& v);
```

*Effects*: Initializes `val` as if direct-non-list-initializing an
object of type `T` with the expression `std::forward<U>(v)`.

*Postconditions*: `bool(*this)` .

*Throws*: Any exception thrown by operations specified in the effect clause.

*Remarks*: If `T`'s selected constructor is a constexpr constructor, this
constructor shall be a constexpr constructor. This constructor shall not
participate in overload resolution unless `T` is not `void` and
`is_constructible_v<T, U&&>` is `true`, `is_same_v<remove_cvref_t<U>, in_place_t>` is
`false`, `is_same_v<expected<T, E>, remove_cvref_t<U>>` is `false`, and
`is_same_v<unexpected<E>, remove_cvref_t<U>>` is `false`. The constructor is explicit if
and only if `is_convertible_v<U&&, T>` is `false`.

```c++
template <class... Args>
    constexpr explicit expected(in_place_t, Args&&... args);
```

*Effects*: Initializes `val` as if direct-non-list-initializing an
object of type `T` with the arguments `std::forward<Args>(args)...` if `T` is not `void`.

*Postconditions*: `bool(*this)` .

*Throws*: Any exception thrown by operations specified in the effect clause.

*Remarks*: If `T`'s constructor selected for the initialization is a constexpr
constructor, this constructor shall be a constexpr constructor. This
constructor shall not participate in overload resolution unless `T` is 
`void` and `sizeof...(Args) == 0` or `T` is not `void` and `is_constructible_v<T, Args...>`.

```c++
template <class U, class... Args>
    constexpr explicit expected(in_place_t, initializer_list<U> il, Args&&... args);
```

*Effects*: Initializes `val` as if direct-non-list-initializing an
object of type `T` with the arguments `il, std::forward<Args>(args)...`.

*Postconditions*: `bool(*this)` .

*Throws*: Any exception thrown by the operations specified in the effect clause.

*Remarks*: If `T`'s constructor selected for the initialization is a constexpr
constructor, this constructor shall be a constexpr constructor. This
constructor shall not participate in overload resolution unless `T` is not
`void` and `is_constructible_v<T, initializer_list<U>&, Args...>`.

```c++
template <class G = E>
    EXPLICIT constexpr expected(const unexpected<G>& e);
```

*Effects*: Initializes `unexpect` as if direct-non-list-initializing
an object of type `unexpected<E>` with the expression `e`.

*Postconditions*: `!bool(*this)`.

*Throws*: Any exception thrown by the operations specified in the effect clause.

*Remark*: If `unexpected<E>`'s selected constructor is a constexpr constructor,
this constructor shall be a constexpr constructor. This constructor shall not
participate in overload resolution unless `is_constructible_v<E, const G&>`. The
constructor is explicit if and only if `is_convertible_v<const G&, E>` is `false`.


```c++
template <class G = E>
    EXPLICIT constexpr expected(unexpected<G>&& e);
```

*Effects*: Initializes `unexpect` as if direct-non-list-initializing
an object of type `unexpected<E>` with the expression `std::move(e)`.

*Postconditions*: `!bool(*this)`.

*Throws*: Any exception thrown by the operations specified in the effect clause.

*Remark*: If `unexpected<E>`'s selected constructor is a constexpr constructor,
this constructor shall be a constexpr constructor. The expression inside
`noexcept` is equivalent to: `is_nothrow_constructible_v<E, G&&>`. This
constructor shall not participate in overload resolution unless
`is_constructible_v<E, G&&>`. The constructor is explicit if and only if
`is_convertible_v<G&&, E>` is `false`.


```c++
template <class... Args>
    constexpr explicit expected(unexpect_t, Args&&... args);
```

*Effects*: Initializes `unexpect` as if direct-non-list-initializing
an object of type `unexpected<E>`with the arguments
`std::forward<Args>(args)...`.

*Postconditions*: `!bool(*this)`.

*Throws*: Any exception thrown by the operations specified in the effect clause.

*Remarks*: If `unexpected<E>`'s constructor selected for the initialization is a
constexpr constructor, this constructor shall be a constexpr constructor. This
constructor shall not participate in overload resolution unless
`is_constructible_v<E, Args&&...>`.

```c++
template <class U, class... Args>
    constexpr explicit expected(unexpect_t, initializer_list<U> il, Args&&... args);
```

*Effects*: Initializes `unexpect` as if direct-non-list-initializing
an object of type `unexpected<E>` with the arguments `il,
std::forward<Args>(args)...`.

*Postconditions*: `!bool(*this)`.

*Throws*: Any exception thrown by the operations specified in the effect clause.

*Remarks*: If `unexpected<E>`'s constructor selected for the initialization is a
constexpr constructor, this constructor shall be a constexpr constructor. This
constructor shall not participate in overload resolution unless
`is_constructible_v<E, initializer_list<U>&, Args&&...>`.

�.�.4.2 Destructor [*expected.object.dtor*] {#expected.object.dtor}
-------------------------------------------

```c++
~expected();
```

*Effects*: If `T` is not `void` and `is_trivially_destructible_v<T> != true` and
`bool(*this)`, calls `val.~T()`. If `is_trivially_destructible_v<E>
!= true` and `!bool(*this)`, calls `unexpect.~unexpected<E>()
`.

*Remarks*: If `T` is `void` or `is_trivially_destructible_v<T>` is `true` and
`is_trivially_destructible_v<E>` is `true` then this destructor shall be a
trivial destructor.

�.�.4.3 Assignment [*expected.object.assign*] {#expected.object.assign}
---------------------------------------------


```c++
expected& operator=(const expected& rhs) noexcept(see below);
```

*Effects*:

If `bool(*this)` and `bool(rhs)`, 

  * assigns `*rhs` to `val` if `T` is not `void`;

otherwise if `!bool(*this)` and `!bool(rhs)`, 

  * assigns `unexpected(rhs.error())` to `unexpect`;

otherwise if `bool(*this)` and `!bool(rhs)`,

  * if `T` is `void`
   
    * initializes `unexpect` as if direct-non-list-initializing an object of type `unexpected<E>` with `unexpected(rhs.error())`. Either
     
      * The didn't throw, set `has_val` to `false`, or
      * the constructor did throw, and nothing was changed.
   
  * otherwise if `is_nothrow_copy_constructible_v<E>`
   
    * destroys `val` by calling `val.~T()`,
    * initializes `unexpect` as if direct-non-list-initializing an object of type `unexpected<E>` with `unexpected(rhs.error())`.

  * otherwise if `is_nothrow_move_constructible_v<E>`

    * constructs a `unexpected<E> tmp` from `*rhs` (this can throw),
    * destroys `val` by calling `val.~T()`,
    * initializes `unexpect` as if direct-non-list-initializing an object of type `unexpected<E>` with `unexpected(rhs.error())`.

  otherwise
   
  * constructs a`T tmp` from `*this` (this can throw),
  * destroys `val` by calling `val.~T()`,
  * initializes `unexpect` as if direct-non-list-initializing an object of type `unexpected<E>` with `unexpected(rhs.error())`. Either,

    * the last constructor didn't throw, set `has_val` to `false`, or
    * the last constructor did throw, so move-construct the `T` from `tmp` back into the expected storage (which can't throw as `is_nothrow_move_constructible_v<T>` is `true`), and rethrow the exception.

otherwise 

  * if `T` is `void` destroys `unexpect` by calling `unexpect.~unexpected<E>()`
  
  * otherwise if `is_nothrow_copy_constructible_v<T>`
    
    * destroys `unexpect` by calling `unexpect.~unexpected<E>() `
    * initializes `val` as if direct-non-list-initializing an object of type `T` with `*rhs`;

  * otherwise if `is_nothrow_move_constructible_v<T>`

    * constructs a `T tmp` from `*rhs` (this can throw),
    * destroys `unexpect` by calling `unexpect.~unexpected<E>() `
    * initializes `val` as if direct-non-list-initializing an object of type `T` with `move(tmp)`;

  * otherwise

    * constructs a `unexpected<E> tmp` from `unexpected(this->error())` (which can throw),
    * destroys `unexpect` by calling `unexpect.~unexpected<E>()`,
    * initializes `val` as if direct-non-list-initializing an object of type `T` with `*rhs`. Either,

      * the constructor didn't throw, set `has_val` to `true`, or
      * the constructor did throw, so move-construct the `unexpected<E>` from `tmp` back into the expected storage (which can't throw as `is_nothrow_move_constructible_v<E>` is `true`), and rethrow the exception.

*Returns*: `*this`.

*Postconditions*: `bool(rhs) == bool(*this)`.

*Throws*: Any exception thrown by the operations specified in the effect clause.

*Remarks*: If any exception is thrown, `bool(*this)` and `bool(rhs)` remain unchanged.

If an exception is thrown during the call to `T`'s or `unexpected<E>`'s copy
constructor, no effect. If an exception is thrown during the call to `T`'s or
`unexpected<E>`'s copy assignment, the state of its contained value is as defined
by the exception safety guarantee of `T`'s or `unexpected<E>`'s copy assignment.

This operator shall be defined as deleted unless 

  * `T` is `void` and `is_copy_assignable_v<E>` and `is_copy_constructible_v<E>` or
  * `T` is not `void` and `is_copy_assignable_v<T>` and `is_copy_constructible_v<T>` and `is_copy_assignable_v<E>` and `is_copy_constructible_v<E>` and (`is_nothrow_move_constructible_v<E>` or `is_nothrow_move_constructible_v<T>`).

```c++
expected& operator=(expected&& rhs) noexcept(see below);
```

*Effects*:

If `bool(*this)` and `bool(rhs)`, 

  * move assign `*rhs` to `val` if `T` is not `void`;

otherwise if `!bool(*this)` and `!bool(rhs)`, 

  * move assign `unexpected(rhs.error())` to `unexpect`;

otherwise if `bool(*this)` and `!bool(rhs)`,

  * if `T` is `void` 

    * initializes `unexpect` as if direct-non-list-initializing an object of type `unexpected<E>` with `unexpected(move(rhs).error())`. Either

      * the constructor didn't throw, set `has_val` to `false`, or
      * the constructor did throw, and nothing was changed.

  * otherwise if `is_nothrow_move_constructible_v<E>`

    * destroys `val` by calling `val.~T()`,
    * initializes `unexpect` as if direct-non-list-initializing an object of type `unexpected<E>` with `unexpected(std::move(rhs.error()))`;

  * otherwise

    * move constructs a `T tmp` from `*this` (which can't throw as `T` is nothrow-move-constructible),
    * destroys `val` by calling `val.~T() `,
    * initializes `unexpect` as if direct-non-list-initializing an object of type `unexpected_type<E>` with `unexpected(std::move(rhs.error()))`. Either,

      * The constructor didn't throw, so mark the expected as holding a `unexpected_type<E>`, or
      * The constructor did throw, so move-construct the `T` from `tmp` back into the expected storage (which can't throw as `T` is nothrow-move-constructible), and rethrow the exception.

otherwise `!bool(*this)` and `bool(rhs)`, 

  * if `T` is `void` destroys `unexpect` by calling `unexpect.~unexpected<E>()`

  * otherwise if `is_nothrow_move_constructible_v<T>`

    * destroys `unexpect` by calling `unexpect.~unexpected<E>()`,
    * initializes `val` as if direct-non-list-initializing an object of type `T` with `*std::move(rhs)`;

  * otherwise

    * move constructs a `unpepected_type<E> tmp` from `unexpected(this->error())` (which can't throw as `E` is nothrow-move-constructible),
    * destroys `unexpect` by calling `unexpect.~unexpected<E>() `,
    * initializes `val` as if direct-non-list-initializing an object of type `T` with `*std::move(rhs)`. Either,

      * The constructor didn't throw, set `has_val` to `true`, or
      * The constructor did throw, so move-construct the `unexpected<E>` from `tmp`  back into the expected storage (which can't throw as `E` is nothrow-move-constructible), and rethrow the exception.

*Returns*: `*this`.

*Postconditions*: `bool(rhs) == bool(*this)`.

*Remarks*: The expression inside noexcept is equivalent to:
`is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>`.

If any exception is thrown, `bool(*this)` and `bool(rhs)` remain
unchanged. If an exception is thrown during the call to `T`'s copy constructor,
no effect. If an exception is thrown during the call to `T`'s copy assignment,
the state of its contained value is as defined by the exception safety guarantee
of `T`'s copy assignment. If an exception is thrown during the call to `E`'s
copy assignment, the state of its contained `unexpect` is as defined by
the exception safety guarantee of `E`'s copy assignment.

This operator shall be defined as deleted unless 

* `T` is `void` and `is_nothrow_move_constructible_v<E>` and `is_nothrow_move_assignable_v<E>`.

or

* `T` is not `void` and `is_move_constructible_v<T>` and `is_move_assignable_v<T>` and `is_nothrow_move_constructible_v<E>` and `is_nothrow_move_assignable_v<E>`.

```c++
template <class U = T>
    expected<T, E>& operator=(U&& v);
```

*Effects*:

If `bool(*this)`, assigns `std::forward<U>(v)` to `val`;

otherwise if `is_nothrow_constructible_v<T, U&&>`

  * destroys `unexpect` by calling `unexpect.~unexpected<E>() `,
  * initializes `val` as if direct-non-list-initializing an object of type `T` with `std::forward<U>(v)` and
  * set `has_val` to `true`;

otherwise

  * move constructs a `unexpected<E> tmp` from `unexpected(this->error())` (which can't throw as `E` is nothrow-move-constructible),
  * destroys `unexpect` by calling `unexpect.~unexpected<E>()`,
  * initializes `val` as if direct-non-list-initializing an object of type `T` with `std::forward<U>(v)`. Either,

      * the constructor didn't throw, set `has_val` to `true`, that is set `has_val` to `true`, or
      * the constructor did throw, so move construct the `unexpected<E>` from `tmp` back into the expected storage (which can't throw as `E` is nothrow-move-constructible), and re-throw the exception.

*Returns*: `*this`.

*Postconditions*: `bool(*this)` .

*Remarks*: If any exception is thrown, `bool(*this)` remains
 unchanged. If an exception is thrown during the call to `T`'s constructor, no
 effect. If an exception is thrown during the call to `T`'s copy assignment, the
 state of its contained value is as defined by the exception safety guarantee of
 `T`'s copy assignment.

This function shall not participate in overload resolution unless:

  * `is_void_v<T>` is `false` and
  * `is_same_v<expected<T,E>, remove_cvref_t<U>>` is `false` and
  * `conjunction_v<is_scalar<T>, is_same<T, decay_t<U>>>` is `false`,
  * `is_constructible_v<T, U>` is `true`,
  * `is_assignable_v<T&, U>` is `true` and
  * `is_nothrow_move_constructible_v<E>` is `true`.

```c++
template <class G = E>
    expected<T, E>& operator=(const unexpected<G>& e);
```

*Effects*:

If `!bool(*this)`, assigns `unexpected(e.error())` to `unexpect`;

otherwise

  * destroys `val` by calling `val.~T()` if `T` is not `void`,
  * initializes `unexpect` as if direct-non-list-initializing an object of type `unexpected<E>` with `unexpected(e.error())` and set `has_val` to `false`.

*Returns*: `*this`.

*Postconditions*: `!bool(*this)`.

*Remarks*: If any exception is thrown, `bool(*this)` remains unchanged.

This signature shall not participate in overload resolution unless
`is_nothrow_copy_constructible_v<E>` and `is_move_assignable_v<E>`.

```c++
expected<T, E>& operator=(unexpected<G> && e);
```

*Effects*:

If `!bool(*this)`, move assign `unexpected(e.error())` to `unexpect`;

otherwise

  * destroys `val` by calling `val.~T()` if `T` is not `void`,
  * initializes `unexpect` as if direct-non-list-initializing an object of type `unexpected<E>` with `unexpected(std::move(e.error()))` and set `has_val` to `false`.

*Returns*: `*this`.

*Postconditions*: `!bool(*this)`.

*Remarks*: If any exception is thrown, `bool(*this)` remains unchanged.

This signature shall not participate in overload resolution unless
`is_nothrow_move_constructible_v<E>` and `is_move_assignable_v<E>`.

```c++
void expected<void,E>::emplace();
```

*Effects*:

If `!bool(*this)` 

  * destroys `unexpect` by calling `unexpect.~unexpected<E>() `,
  * set `has_val` to `true`

*Postconditions*: `bool(*this)` .

*Throws*: Nothing

```c++
template <class... Args>
    void emplace(Args&&... args);
```

*Effects*:

If `bool(*this)`, assigns `val` as if
constructing an object of type `T` with the arguments
`std::forward<Args>(args)...`

otherwise if `is_nothrow_constructible_v<T, Args&&...>`

  * destroys `unexpect` by calling `unexpect.~unexpected<E>() `,
  * initializes `val` as if direct-non-list-initializing an object of type `T` with `std::forward<Args>(args)...` and
  * set `has_val` to `true`;

otherwise if `is_nothrow_move_constructible_v<T>`

  * constructs a `T tmp` from `std::forward<Args>(args)...` (which can throw),
  * destroys `unexpect` by calling `unexpect.~unexpected<E>()`,
  * initializes `val` as if direct-non-list-initializing an object of type `T` with `std::move(tmp)` (which can not throw) and
  * set `has_val` to `true`;
   
otherwise

  * move constructs a `unexpected<E> tmp` from `unexpected(this->error())` ,
  * destroys `unexpect` by calling `unexpect.~unexpected<E>()`,
  * initializes `val` as if direct-non-list-initializing an object of type `T` with `std::forward<Args>(args)...`. Either,

      * the constructor didn't throw, set `has_val` to `true`, that is set `has_val` to `true`, or
      * the constructor did throw, so move-construct the `unexpected<E>` from `tmp` back into the expected storage (which can't throw as `E` is nothrow-move-constructible), and re-throw the exception.

*Postconditions*: `bool(*this)` .

*Throws*: Any exception thrown by the operations specified in the effect clause.

*Remarks*: If an exception is thrown during the call to `T`'s assignment, nothing
changes.

This signature shall not participate in overload resolution unless
`is_nothrow_constructible_v<T, Args&&...>`.


```c++
template <class U, class... Args>
    void emplace(initializer_list<U> il, Args&&... args);
```

*Effects*: if `bool(*this)`, assigns `val` as if
constructing an object of type `T` with the arguments `il,
std::forward<Args>(args)...`, otherwise destroys `unexpect` by calling
`unexpect.~unexpected<E>()` and initializes `val` as if
constructing an object of type `T` with the arguments `il,
std::forward<Args>(args)...`.

*Postconditions*: `bool(*this)` .

*Throws*: Any exception thrown by the operations specified in the effect clause.

*Remarks*: If an exception is thrown during the call to `T`'s assignment nothing
changes.

The function shall not participate in overload resolution unless:
`T` is not `void` and `is_nothrow_constructible_v<T, initializer_list<U>&, Args&&...>`.

�.�.4.4 Swap [*expected.object.swap*] {#expected.object.swap}
-------------------------------------

```c++
void swap(expected<T, E>& rhs) noexcept(see below);
```

Issue: Check `swap` Effects.

*Effects*: if `bool(*this)` and `bool(rhs)`, 

  * calls `using std::swap; swap(val, rhs.val)`, 
    
otherwise if `!bool(*this)` and `!bool(rhs)`, 

  * calls `using std::swap; swap(unexpect, rhs.unexpect)`, 

otherwise if `!bool(*this)` and `bool(rhs)`,

  * calls `rhs.swap(*this)`, 

otherwise

  * if `T` is `void`
  
    * initializes `unexpect` as if direct-non-list-initializing an object of type `unexpected<E>` with `unexpected(std::move(rhs))`. Either
    
      * the constructor didn't throw, set `has_val` to `false`, destroys `unexpect` by calling `rhs.unexpect.~unexpected<E>()` set `rhs.has_val` to `true`.
      * the constructor did throw, rethrow the exception.

  * otherwise if `is_nothrow_move_constructible_v<E>`, 
   
    * the `unexpect` of `rhs` is moved to a temporary variable `tmp` of type `unexpected_type`, 
    * followed by destruction of `unexpect` as if by `rhs.unexpect.unexpected<E>::~unexpected<E>()`,
    * `rhs.val` is direct-initialized from `std::move(*other)`, 
    * followed by destruction of `rhs.val` as if by `rhs.val->~T()`,
    * the `unexpect` of `this` is direct-initialized from `std::move(tmp)`, after this, `this` does not contain a value; and `bool(rhs)` . 

  * otherwise if `is_nothrow_move_constructible_v<T>`, 

    * `rhs.val` is moved to a temporary variable `tmp` of type `T`, 
    * followed by destruction of `rhs.val` as if by `rhs.val.~T()`,
    * the `unexpect` of `rhs` is direct-initialized from `unexpected(std::move(other.error()))`, 
    * followed by destruction of `unexpect` as if by `rhs.unexpect->unexpected<E>::~unexpected<E>()`,
    * `val` is direct-initialized from `std::move(tmp)`, after this, `this` does not contain a value; and `bool(rhs)` . 

*Throws*: Any exceptions that the expressions in the Effects clause throw.

Issue: Adapt `swap` Remarks once Effects are good.

*Remarks*: The expression inside noexcept is equivalent to:
`is_nothrow_move_constructible_v<T> && noexcept(swap(declval<T&>(),
declval<T&>())) && is_nothrow_move_constructible_v<E> &&
noexcept(swap(declval<E&>(), declval<E&>()))`. The function shall not
participate in overload resolution unless: Lvalues of type `T` shall be
`Swappable`, Lvalues of type `E` shall be `Swappable` and
`is_move_constructible_v<E>` `is_move_constructible_v<E>`
`is_move_constructible_v<E>` or `is_move_constructible_v<T>`.

�.�.4.5 Observers [*expected.object.observe*] {#expected.object.observe}
---------------------------------------------

```c++
constexpr const T* operator->() const;
T* operator->();
```

*Requires*: `bool(*this)` .

*Returns*: `addressof(val)`.

*Remarks*: Unless `T` is a user-defined type with overloaded unary `operator&`,
the first operator shall be a constexpr function. 
The operator shall not participate in overload resolution unless: `T` is not `void`. 

```c++
constexpr const T& operator *() const&;
T& operator *() &;
```

*Requires*: `bool(*this)` .

*Returns*:
`val`.

*Remarks*: The first operator shall be a constexpr function. 
The operator shall not participate in overload resolution unless: `T` is not `void`.

```c++
constexpr T&& operator *() &&;
constexpr const T&& operator *() const&&;
```

*Requires*: `bool(*this)` .

*Returns*: `std::move(val)`.

*Remarks*: This operator shall be a constexpr function.
The operator shall not participate in overload resolution unless: `T` is not `void`.

```c++
constexpr explicit operator bool() noexcept;
```

*Returns*: `has_val`.

*Remarks*: This operator shall be a constexpr function.

```c++
constexpr bool has_value() const noexcept;
```

*Returns*: `has_val`.

*Remarks*: This function shall be a constexpr function.

```c++
constexpr void expected<void, E>::value() const;
```

*Throws*: `bad_expected_access(error())` if `!bool(*this)`.


```c++
constexpr const T& expected::value() const&;
constexpr T& expected::value() &;
```

*Returns*: `val`, if `bool(*this)` .

*Throws*: `bad_expected_access(error())` if `!bool(*this)`.

*Remarks*: These functions shall be constexpr functions.
The operator shall not participate in overload resolution unless: `T` is not `void`.


```c++
constexpr T&& expected::value() &&;
constexpr const T&& expected::value() const&&;
```

*Returns*: `std::move(val)`, if `bool(*this)` .

*Throws*: `bad_expected_access(error())` if `!bool(*this)`.

*Remarks*: These functions shall be constexpr functions.
The operator shall not participate in overload resolution unless: `T` is not `void`.

```c++
constexpr const E& error() const&;
constexpr E& error() &;
```

*Requires*: `!bool(*this)`.

*Returns*: `unexpect.value()`.

*Remarks*: The first function shall be a constexpr function.

```c++
constexpr E&& error() &&;
constexpr const E&& error() const &&;
```

*Requires*: `!bool(*this)`.

*Returns*: `std::move(unexpect.value())`.

*Remarks*: The first function shall be a constexpr function.


```c++
template <class U>
    constexpr T value_or(U&& v) const&;
```

*Effects*: Equivalent to `return bool(*this) ? **this :
 static_cast<T>(std::forward<U>(v));`.

*Remarks*: If `is_copy_constructible_v<T> && is_convertible_v<U&&, T>` is
 `false` the program is ill-formed.

```c++
template <class U>
    constexpr T value_or(U&& v) &&;
```

*Effects*: Equivalent to `return bool(*this) ? std::move(**this) :
static_cast<T>(std::forward<U>(v));`.

*Remarks*: If `is_move_constructible_v<T>` and `is_convertible_v<U&&, T>` is
 `false` the program is ill-formed.

�.�.5 `unexpect` tag [*expected.unexpect*] {#expected.unexpect}
------------------------------------------

```c++
struct unexpect_t {
    explicit unexpect_t() = default;
};
inline constexpr unexpect_t unexpect{};
```

�.�.6 Template Class `bad_expected_access` [*expected.bad_expected_access*] {#expected.bad_expected_access}
---------------------------------------------------------------------------

```c++
template <class E>
class bad_expected_access : public bad_expected_access<void> {
public:
    explicit bad_expected_access(E);
    virtual const char* what() const noexcept override;
    E& error() &;
    const E& error() const&;
    E&& error() &&;
    const E&&  error() const&&;
private:
    E val; // exposition only
};
```

Issue: Wondering if we just need an `const &` overload as we do for `system_error`.

The template class `bad_expected_access` defines the type of objects thrown as
exceptions to report the situation where an attempt is made to access the value
of `expected` object that contains an `unexpected<E>`.

```c++
bad_expected_access::bad_expected_access(E e);
```

*Effects*: Initialize `val` with `e`.

*Postconditions*: `what()` returns an implementation-defined NTBS.

```c++
const E& error() const&;
E& error() &;
 ```

*Effects*: Equivalent to: `return val;`


```c++
E&& error() &&;
const E&& error() const &&;
 ```

*Effects*: Equivalent to: `return std::move(val);`


```c++
virtual const char* what() const noexcept override;
```

*Returns*: An implementation-defined NTBS.

�.�.7 Class `bad_expected_access<void>` [*expected.bad_expected_access_base*] {#expected.bad_expected_access_base}
--------------------------------------------------------------------------------------

```c++
template <>
class bad_expected_access<void> : public exception {
public:
    explicit bad_expected_access();
};
```



�.�.8 Expected Equality operators [*expected.relational_op*] {#expected.relational_op}
--------------------------------------------------------------

```c++
template <class T1, class E1, class T2, class E2>
    constexpr bool operator==(const expected<T1, E1>& x, const expected<T2, E2>& y);
```

*Requires*: The expressions `*x == *y` and
`unexpected(x.error()) == unexpected(y.error())` shall be well-formed and its result
shall be convertible to `bool`. 

*Returns*: If `bool(x) != bool(y)`, `false`; otherwise if `bool(x) == false`,
`x.error() == y.error()`; otherwise `true` if `T` is
`void` or `*x == *y` otherwise.

*Remarks*: Specializations of this function template, for which `T` is `void` or
`*x == *y` and `x.error() == y.error()` are core constant expression, shall be
`constexpr` functions.

```c++
template <class T1, class E1, class T2, class E2>
    constexpr bool operator!=(const expected<T1, E1>& x, const expected<T2, E2>& y);
```

*Requires*: The expressions `*x != *y` and `x.error() != y.error()` shall be
well-formed and its result shall be convertible to `bool`.

*Returns*: If `bool(x) != bool(y)`, `true`; otherwise if `bool(x) == false`,
`x.error() != y.error()`; otherwise `true` if `T` is
`void` or `*x != *y`.

*Remarks*: Specializations of this function template, for which `T` is `void` or
`*x != *y` and `x.error() != y.error()` are core
constant expression, shall be constexpr functions.

�.�.9 Comparison with `T` [*expected.comparison_T*] {#expected.comparison_T}
---------------------------------------------------

```c++
template <class T, class E, class U> constexpr bool operator==(const expected<T, E>& x, const U& v);
template <class U, class T, class E> constexpr bool operator==(const U& v, const expected<T, E>& x);
```

*Requires*: `T` is not `void` and the expression `*x == v` shall be well-formed
and its result shall be convertible to
`bool`. [ *Note:* `T` need not be *EqualityComparable*. - *end note*]

*Effects*: Equivalent to: `return bool(x) ? *x == v : false;`.


```c++
template <class T, class E, class U> constexpr bool operator!=(const expected<T, E>& x, const U& v);
template <class U, class T, class E> constexpr bool operator!=(const U& v, const expected<T, E>& x);
```

*Requires*: `T` is not `void` and the expression `*x == v` shall be well-formed
and its result shall be convertible to
`bool`. [ *Note:* `T` need not be *EqualityComparable*. - *end note*]

*Effects*: Equivalent to: `return bool(x) ? *x != v : false;`.


�.�.10 Comparison with `unexpected<E>` [*expected.comparison_unexpected_E*] {#expected.comparison_unexpected_E}
---------------------------------------------------------------------------

```c++
template <class T, class E, class G> constexpr bool operator==(const expected<T, E>& x, const unexpected<G>& e);
template <class G, class T, class E> constexpr bool operator==(const unexpected<G>& e, const expected<T, E>& x);
```

*Requires*: The expression `unexpected(x.error()) == e` shall be well-formed and
its result shall be convertible to `bool`. 

*Effects*: Equivalent to: `return bool(x) ? false : unexpected(x.error()) == e;`.

```c++
template <class T, class E, class G> constexpr bool operator!=(const expected<T, E>& x, const unexpected<G>& e);
template <class G, class T, class E> constexpr bool operator!=(const unexpected<G>& e, const expected<T, E>& x);
```

*Requires*: The expression `unexpected(x.error()) != e` shall be well-formed and
its result shall be convertible to `bool`. 
*Effects*: Equivalent to: `return bool(x) ? true : unexpected(x.error()) != e;`.


�.�.11 Specialized algorithms [*expected.specalg*] {#expected.specalg}
--------------------------------------------------

```c++
template <class T, class E>
void swap(expected<T, E>& x, expected<T, E>& y) noexcept(noexcept(x.swap(y)));
```

*Effects*: Calls `x.swap(y)`.

*Remarks*: This function shall not participate in overload resolution unless `T`
is `void` or `is_move_constructible_v<T>` is `true`, `is_swappable_v<T>` is
`true` and `is_move_constructible_v<E>` is `true` and `is_swappable_v<E>` is
`true`.


Open Questions {#qq}
==============

`std::expected` is a vocabulary type with an opinionated design and a proven
record under varied forms in a multitude of codebases. Its current form has
undergone multiple revisions and received substantial feedback, falling roughly
in the following categories:

  1. **Ergonomics**: is this *the right way* to expose such functionality?
  1. **Disappointment**: should we expose this in the Standard, given C++'s
     existing error handling mechanisms?
  1. **STL usage**: should the Standard Template Library adopt this class, at
     which pace, and where?

LEWG and EWG have nonetheless reached consensus that a class of this general
approach is probably desirable, and the only way to truly answer these questions
is to try it out in a TS and ask for explicit feedback from developers. The
authors hope that developers will provide new information which they'll be able
to communicate to the Committee.

Here are open questions, and questions which the Committee thinks are settled
and which new information can justify revisiting.

Ergonomics {#ergo}
----------

  1. Name: 
  
    * Is `expected` the right name?
    * Does it express intent both as a consumer and a producer?

  1. Is `E` a salient property of `expected`?
  1. Is `expected<void, E>` clear on what it expresses as a return type?
  1. Would it make sense for `expected` to support containing *both* `T` and `E` (in some designs, either one of them being optional), or is this use case better handled by a separate proposal?
  1. Is the order of parameters `<T, E>` appropriate?
  1. Is usage of `expected` "viral" in a codebase, or can it be adopted incrementally?
  1. Comparisons:

    * Are `==` and `!=` useful?
    * Should other comparisons be provided?
    * What usages of `expected` mandate putting instances in a `map`, or other such container?
    * Should `hash` be provided?
    * What usages of `expected` mandate putting instances in an `unordered_map`, or other such container?
    * Should `expected<T, E>` always be comparable if `T` is comparable, even if `E` is not comparable?
      
  1. Error type `E`:
  
    * `E` has no default. Should it?
    * Should `expected` be specialized for particular `E` types such as `exception_ptr` and how?
    * Should `expected` handle `E` types with a built-in "success" value any differently and how?
    * `expected` is not implicitly constructible from an `E`, even when unambiguous from `T`, because as a vocabulary type it wants unexpected error construction to be verbose, and require hopping through an `unexpected`. Is the verbosity extraneous?
      
  1. Does usage of this class cause a meaningful performance impact compared to using error codes?
  1. The accessor design offers a terse unchecked dereference operator (expected to be used alongside the implicit `bool` conversion), as well as `value()` and `error()` accessors which are checked. Is that a gotcha, or is it similar enough to classes such as `optional` to be unsurprising?
  1. Is `bad_expected_access` the right thing to throw?
  1. Should some members be `nodiscard`?


Disappointment {#dis}
--------------

C++ already supports exceptions and error codes, `expected` would be a third
kind of error handling.

  1. where does `expected` work better than either exceptions or error handling?
  1. `expected` was designed to be particularly well suited to APIs which require their immediate caller to consider an error scenario. Do it succeed in that purpose?
  1. Do codebases successfully compose these three types of error handling?
  1. Is debuggability any harder?
  1. Is it easy to teach C++ as a whole with a third type of error handling?
     

STL Usage {#stl}
---------

  1. Should `expected` be used in the STL at the same time as it gets standardized?
  1. Where, considering `std2` may be a good place to change APIs?
